Can a doctor tell if a girl is virgin or not?

Not always. If there is an intact hymen, then the person is unlikely to have had vaginal sex. However, the anatomy of hymen varies from imperforate hymen to lack of hymen. For info: https://www.who.int/news/item/17-10-2018-united-nations-agencies-call-for-ban-on-virginity-testing#:~:text=Virginity%20testing%20is%20often%20performed,woman's%20history%20of%20sexual%20activity.

If you come in with a STD involving your genitals, they will assume that is how you got it. Since some women are born with no hymen or have it opened in a childhood injury, they would not be sure unless you told them, which you should always.